The static pressure for the expansion tank should be about the same as you 'normal' pressure reading. So, if the incoming water pressure is around 60#, it's fine. If you take a reading after it is in the system, it will be whatever the water pressure is...to test and change, you need to relieve the water pressure on the bladder first, then adjust. If it's balanced with the static water pressure, there's very little movement of the bladder. Otherwise, it is potentially stretched further than needed, and it might wear out faster. Orientation doesn't matter except when it fails, with it sticking up, you'll drain all of the water forced into it all over. Doesn't happen often, but keep that in mind. The tank should be supported so if it does fail and fill with water, the stress to the piping and fittings isn't great.

As a practical matter, ANY pressure above zero and below 150 will accomplish the purpose to some extent. The closer it is to the normal pressure in the system the better it works. If the initial pressure is less than the normal pressure, it WILL stabilize at the house pressure as soon as you turn the water valve on.